The present invention relates to a laser strainmeter for observing small strain changes of a medium, e.g., the earth's crust, by measuring changes of diameters of a cylindrical or spherical container buried in the medium.
Precise observation of small strain changes of the earth's crust is important to studies of solid earth science or for practical purposes such as earthquake prediction or prediction of a volcanic eruption. In order to precisely measure small strain changes of the earth's crust, it is important to avoid the influence of change of temperature on the surface of the earth, or the effect of rain or an atmospheric pressure change. Accordingly, the conventional practice is to dig a borehole in the ground so deep as to reach bedrock and to install a strainmeter to the bottom of the borehole with cement or mortar.
Strainmeters of the type described above include Sakata-type borehole three-component strainmeter (Shoji SAKATA AND Haruo SATO, BOREHOLE-TYPE TILTMETER AND THREE-COMPONENT STRAINMETER FOR EARTHQUAKE PREDICTION, J. Phys. Earth, 34, Suppl., S129-S140, 1986). According to the Sakata-type borehole three-component strainmeter, deformation of a cylindrical container buried in a borehole is converted into volume changes of liquid contained in chambers of the container, and the volume changes are electrically detected. The Sakata-type borehole three-component strainmeter provides high resolution, and since it has minimal portions which involve solid friction, the apparatus is not affected by the shock of an earthquake. Accordingly, the strainmeter has the advantage of high stability. There is another borehole-type three-component strainmeter which is based on changes of bore diameters. For example, changes in diameter in three directions are enlarged by a combination of mechanical levers, and detected in the form of electric signals.
For future more effective earthquake prediction and more detailed observational studies in earth science, there is a need for a strainmeter which provides higher resolution than the conventional three-component strainmeter as described above. On the other hand, a strainmeter which is capable of operating under high-temperature conditions is demanded for prediction of volcanic eruptions. Further, there is an increasing demand for measurement in a borehole as deep as 10 km or more, which is attained by ultra-deep boring, and a strainmeter capable of enduring high temperature at that depth is demanded. Further, when it is considered to use a strainmeter as a seismometer for detecting a seismic wave, the strainmeter must function accurately in a high-frequency region.
However, the above-described conventional borehole-type three-component strainmeter suffers from the problem that, since it uses a liquid, phase change takes place at high temperature, causing the strainmeter to fail to function as desired. Further, the strainmeter cannot detect short period phenomena such as seismic waves because of the viscous resistance in the flow path. In addition, the conventional borehole-type three-component strainmeter requires a multicore cable. In view of installation in deeper boreholes or on the bottom of the sea, it is preferable to minimize the number of conductors constituting the cable for easy handling. In this case, the number of conductors of the cable is reduced by introducing a carrier device. However, it is desirable not to use a carrier device because carrier devices are generally incapable of withstanding high temperature. The existing borehole-type three-component strainmeter further involves the problem that since the overall length is long for reasons of structure, if it is combined with other observation devices to form an integrated observation system, the overall length becomes undesirably longer. The above-described mechanical borehole-type three-component strainmeter involves solid friction and backlash. Therefore, the degree of accuracy is inevitably low, and it cannot withstand a shock in particular.